Modern wireless communication techniques make use of RF signals which are modulated both in phase and in amplitude. This allows for a significant increase in the transmission speed, i.e., the amount of transferred information per unit of time, without having to increase the bandwidth occupation.
Generally, the power of a signal transmitted by a transmitting apparatus has to be regulated based on the distance the signal has to travel for reaching the receiving apparatus. Therefore, practically the totality of all the modern radio transmitting apparatuses are provided with at least one amplifier circuit, referred to as power amplifier.
As it is well known to those skilled in the art, employing proper power amplifier topologies, it is possible to easily amplify a signal that is modulated only in phase with a high efficiency, i.e., using a relatively low amount of power. However, the amplification of an amplitude modulated signal is more difficult. Indeed, in order to amplify an amplitude modulated signal, known solutions provide for employing power amplifiers of the linear type, which are characterized by very low efficiencies.
In order to increase the efficiency of the amplification of an amplitude modulated RF signal, one prior approach provides a power amplifier adapted to be employed in the broadcast transmitter apparatuses for the (now obsolete) AM radio stations. In more detail, such a power amplifier includes a plurality of small (identical) elementary amplifiers having the same gain. The elementary amplifiers have inputs connected to an RF source (the carrier) and outputs connected to the primary windings of a transformer. The load of the transmitter apparatus (typically, an antenna) is connected to the secondary winding of such transformer. The turning on and turning off of each single elementary amplifier is determined by a respective control signal. The control signals are generated through a digitalization process of a further input signal containing the information relating the desired amplitude modulation to be impressed on the carrier. As a consequence, the amplitude of the output carrier is essentially proportional to the number of elementary amplifiers which are turned on, and thus to the value of the input signal.
Since the elementary amplifiers which are off dissipate a very small amount of power, the efficiency of the power amplifier is increased with respect to the previous solutions. In order to increase the resolution of the amplitude modulation, i.e., in order to improve the granularity with which the carrier amplitude may be defined, the power amplifier may be further provided with additional elementary amplifiers, each one having a gain equal to a respective fraction (e.g., ½, ¼, ⅛ and so on) of the gain of the previously described identical elementary amplifiers. Each elementary amplifier can include a full-bridge transistor circuit and an input transformer. In order to eliminate such input transformers, proper driving circuits can be included between the source of the RF carrier and the elementary amplifiers. In this way, it is possible to eliminate the input transformers, reducing manufacturing costs and the volume of the whole device. Moreover, with this solution, the power consumption is strongly reduced since the power required for driving the full-bridge transistor circuits may be entirely provided by the driving circuits.
Unfortunately, the above-mentioned solutions do not lend themselves to implementation in integrated circuits. Moreover, these solutions are not adapted to correctly operate at the frequencies used in modern wireless networks (0.7-5.8 GHz) because of the presence of the output transformer which is provided with a high number of windings.
Digital amplitude modulators adapted to be implemented in an integrated circuit can include an array of MOS controlled switches. The controlled switches are connected to an oscillator, which represents the RF source. The controlled switches are driven through a respective group of control bits, representing the desired amplitude. Thus, the amplitude of the output carrier is directly controlled by the value assumed by the control bits.
Although the above-mentioned solution is adapted to be implemented in an integrated circuit in order to reach the requested level of power, a further output amplifier is required. Moreover, the signal provided by the oscillator has to be amplified by a buffer amplifier, too, in order to efficiently drive the switches. Increasing the size of the controlled switches for draining a higher amount of power would require a corresponding increasing in the size of such buffer amplifier, reducing the global efficiency of the modulator.
A further limit of known digital amplitude modulators regards the field of code-multiplexed transmissions (such as in the CDMA and WCDMA transmission standards). Indeed, in these cases it is required that the average power transmitted from the mobile terminal is controllable over a wide range of values. For example, in a transmission following the WCDMA standard, the average power of the modulated carrier has to be varied by a factor higher than 10000000, i.e., higher than 70 dB. Known digital amplitude modulators cannot generate a signal having such a high power range, since the power generated by the smallest elementary amplifier would be exceptionally lower than the total power that can be generated.
It is thus desirable to have a digital amplitude modulator which can control the average power, and which is able to deliver a sufficiently high level of power for exploitation in a wireless mobile terminal.